I have used serveral of these vintage original accessory Manifold Heaters, and yes there were several different makes, and the fitting to the firewall is a concern to be aware of.
However, in-between the end of the heater and the firewall, there is a space that needs to be filled, which connects the heater to the firewall. It generally is a piece that can (if carefully selected) be used to 'adjust' for any mis-alignment, if not too far amiss.
Flexible Aluminum Ducting is what I have most often used to adjust for the mis-alignment. It will probably work on 80% of the ill-fitting heater/hole alignments.
And, oh yes, a heater on an Open Car DOES make sense - especially if that is the only car you have. For short trips in 'coolish' weather, a Racoon-Skin coat will work fine. But on longer trips in COLD weather, with a full set of Side-Curtains to enshroud your cocoon, my Otwell Health Heater worked wonders !
- Doug Vieyra, Eureka, California